1. Field of the Invention
This invention relates to fuel cells and more particularly to a matrix for retaining the electrolyte in a phosphoric acid fuel cell.
2. Description of the Prior Art
One type of fuel cell which is well known in the art is a fuel cell using liquid phosphoric acid as the electrolyte retained in a matrix disposed between a pair of gas diffusion electrodes. Typically, a hydrogen containing gas is used as the fuel and an oxygen containing gas, such as air, is used as the oxidant. The electrodes may comprise, for example, a catalyst layer of platinum black mixed with polytetrafluoroethylene, the catalyst layer being disposed on a support of carbon paper impregnated with a wet proofing hydrophobic polymer such as polytetrafluoroethylene.
In order for the electrolyte retaining matrix to perform satisfactorily in this type of cell it must have the following properties: (1) It must be porous and have good liquid permeability; (2) it must be wettable to the electrolyte and provide good ionic conductivity; (3) it must be an electronic insulator; (4) it must be chemically stable in the phosphoric acid electrolyte at fuel cell operating temperatures and at electrode open circuit potentials such that even after 40,000 hours of operation there are no significant generation of materials that will poison the catalyst; (5) it must provide a bubble pressure sufficient to prevent reactant gas crossover; and (6) it should be as thin as possible yet still provide all of the foregoing properties.
One matrix material which has been used in the past is phenolic resin, such as described in U.S. Pat. No. 3,694,310 to Emanuelson et al of common assignee with the present invention. The difficulty with a phenolic resin type of matrix (and matrices made of other types of organic fibers or powders) is that over a long period of time there is a reaction between the phosphoric acid and the organic material at temperatures greater than about 250.degree. F. The reaction produces a molecule which adsorbs onto the electrode catalyst and poisons the catalyst, resulting in performance degradation. Since it is desirable that the basic fuel cell component in a commercial fuel cell power plant operate satisfactorily for 40,000 hours or more without requiring replacement, even a very slow reaction between the acid and the matrix can be quite harmful.
Many other materials, both organic and inorganic have been suggested for use as an acid fuel cell matrix, but all have been deficient in one or more respects, such as by corroding, going into solution with the phosphoric acid, not providing the necessary electronic insulation, or being deficient in such areas as wettability, porosity, or bubble pressure.
Adlhart et al U.S. Pat. No. 3,575,718 discusses the many properties required for a phosphoric acid fuel cell electrolyte member. Adlhart el al found it necessary to construct an electrolyte retaining member of more than one layer, each layer comprising different materials in order that the deficiencies of one layer can be either nullified or provided by the properties in the other layer. This type of matrix construction is expensive and has its limitations.
Presently no prior art phosphoric acid electrolyte matrix material satisfies all of the foregoing properties, is inexpensive, and has the desired long life necessary for commercial fuel cell power plants.